This invention relates generally to optical transparencies, and, more particularly, to optical transparencies having a laminated, electrically-conductive layer for shielding against electromagnetic interference (EMI) and electromagnetic pulses (EMP).
Many aircraft now being built and flown must be capable of protecting their sensitive electronic equipment from the harmful effects of EMI and EMP. EMP, which are created by lightning and nuclear explosions, can have an intensity as high as 50 kilovolts per meter, in a frequency range of about 10 kilohertz to 100 megahertz. For intensities this high, substantial shielding is essential.
In the past, optical transparencies of such aircraft have protected against EMI and EMP by including laminated, electrically-conductive screens, frequently carried on the inwardly-facing surface of a plastic (e.g., polymethyl methacrylate) sheet. The screens typically have taken the form of a simple X-Y grid of electrical conductors that define an array of small, square apertures. The laminated screens are highly effective in substantially attenuating EMI and EMP, yet they transmit a very high percentage (e.g. 95%) of visible light, with little distortion.
Although the optical transparencies of the kind described above have proven to be highly effective in attenuating EMI and EMP, with good transmission of visible light, the transparencies can sometimes provide objectionable distortion to the visible light, particularly point sources of light and particularly at night. Fraunhofer diffraction can cause a point source of light to appear to have a generally cross shape, with orthogonal streaks emanating from the location of the actual point source. The streaks are aligned with the two axes of the grid shield and can be highly distracting to a person viewing through the transparency, particularly when multiple point sources of light (e.g. landing strip lights) are being viewed.
Diffraction-induced distractions of the kind described above can be eliminated by using a grid that defines a compact array of circular holes, like the grids in the doors of many microwave ovens. However, such grids are considered to transmit too low a proportion of incident visible light for many applications, particularly aircraft applications.
It should, therefore, be appreciated that there is a need for an improved optical transparency that shields against EMI and EMP, yet that transmits a high proportion of incident visible light and that provides reduced distraction caused by Fraunhofer diffraction. The present invention fulfills this need.